1. Field of the Invention
One of the problems associated with holograms is that diffraction optics display systems utilizing a hologram are degraded by the effects of the spurious reflection and transmission hologram recordings that are generated during the holographic exposure process. These spurious holograms have been found to be generated by reflections from surfaces which are interfaces of materials of differing index of refraction such as
B air-glass interfaces of the transparent surfaces of the recording cover plate, the substrate, the recording medium such as the gelatin or optical elements. These reflections combine with the primary holographic beams at the recording film to form both spurious reflection hologram recordings and spurious transmission hologram recordings. In a display system utilizing a hologram, the spurious reflection hologram recordings create ghost images and the spurious transmission hologram recordings create rainbow-like flare patterns. The purpose of this invention is to prevent the recording of the unwanted spurious holograms that are caused by reflections and thus to eliminate both ghost and flare images in the final display. The invention is directed to forming reflection type holograms from a single coherent beam such as a laser beam with elimination of undesired hologram recordings that result from spurious reflections. 2. Description of the Prior Art
Prior to this invention there was no known method to reduce the flare and ghost images of holograms to acceptable levels. These undesired images were of an especially high undesirable level in holograms formed from energy sources positioned at broad angles. The basic approaches that have been utilized in an attempt to minimize the problem are either to anti-reflection coat all reflective surfaces of the hologram forming system, to embed all optical elements in an index matching fluid when forming the hologram such that no air to glass interface reflections reach the hologram area, or form the hologram with the beams impinging the film supporting elements at Brewster's angle. The first method has been found to be inadequate because the best known anti-reflection coating provides results that are many times worse than required. The second method is generally unsuccessfull because the striations in large fluid volumes degrade the holographic recording process. A third approach of putting all reflective surfaces at Brewster's angle is only possible in very simple systems such as those in which flat optics and collimated beams are used. The third method is unsatisfactory because of the restriction of the angular placements of the recording, and hence playback, beams of the recording medium.